Instrument landing systems (ILSs) provide navigation guidance to landing aircraft. Generally, an ILS includes a localizer to provide lateral navigation and a glidescope to provide guidance for descent to a runway threshold. Both the localizer and the glidescope emit signals, which are received by incoming aircraft and translated to navigation information.
The signals of prior art ILSs are subject to interference from a variety of sources including multipath. Multipath occurs when the transmitted signals travel between the ground-based transmitter and airborne receiver via multiple paths due to reflections or diffractions of the signal. Such multipath interference may occur when an aircraft is taking off and flies over the localizer, thereby becoming a source of reflection or refraction of the localizer signal received by another airplane on approach. When ILS signals are received via multiple paths, the signals add constructively or destructively at the receiver antenna resulting in a distortion of the guidance signal. In this way, multipath interference in ILS signals received by prior art ILS receivers in aircraft can cause the ILS signals to appear as though the aircraft is moving laterally with respect to the runway centerline even if no such lateral movement of the aircraft is occurring. Accordingly, there is a need for mitigation of ILS overflight interference.